In Search of Crater Chains

May
15, 2006: As the fragments of shattered comet 73P/Schwassmann
Wachmann 3 glide harmlessly past Earth this month
in full view of backyard telescopes, onlookers can't help
but wonder, what if a comet like that didn't miss, but actually
hit our planet?

For
the answer to that question, we look to the Sahara desert.

In
a remote windswept area named Aorounga, in Chad, there are
three craters in a row, each about 10 km in diameter. "We
believe this is a 'crater chain' formed by the impact of a
fragmented comet or asteroid about 400 million years ago in
the Late Devonian period," explains Adriana Ocampo of
NASA headquarters.

Ocampo
and colleagues discovered the chain in 1996. The main crater
"Aorounga South" had been known for many years—it
sticks out of the sand and can be seen from airplanes and
satellites. But a second and possibly third crater were buried.
They lay hidden until radar onboard the space shuttle (SIR-C)
penetrated the sandy ground, revealing their ragged outlines.

"Here
on Earth, crater chains are rare," says Ocampo, but they
are common in other parts of the solar system.

The first crater chains were discovered by NASA's Voyager
1 spacecraft. In 1979 when the probe flew past Jupiter's moon
Callisto, cameras recorded a line of craters, at least fifteen
long, evenly spaced as if someone had strafed the moon with
a Gatling gun. Eventually, eight chains were found on Callisto
and three more on Ganymede.

At
first the chains were a puzzle. Were they volcanic? Had an
asteroid skipped along the surface of Callisto like a stone
skipping across a pond?

The
mystery was solved in 1993 with the discovery of Comet Shoemaker-Levy
9. SL-9 was not a single comet, but a "string of pearls,"
a chain of 21 comet fragments created a year earlier when
Jupiter's gravity ripped the original comet apart. SL-9 struck
back in 1994, crashing into Jupiter. Onlookers watched titanic
explosions in the giant planet's atmosphere, and it only took
a little imagination to visualize the result if Jupiter had
had a solid surface: a chain of craters.

Astronomers
have since realized that fragmented comets and rubble-pile
asteroids are commonplace. Comets fall apart rather easily;
sunlight alone can shatter their fragile nuclei. Furthermore,
there is mounting evidence that many seemingly solid asteroids
are assemblages of boulders, dust and rock held together by
feeble gravity. When these things hit, they make chains.

In
1994, researchers Jay Melosh and Ewen Whitaker announced their
finding of two crater chains on the Moon. One, on the floor
of the crater Davy, is spectacular--an almost perfect line
of 23 pockmarks each a few miles in diameter. This proved
that crater chains exist in the Earth-Moon system.

But
where on Earth are they?

Earth
tends to hide its craters. "Wind and rain erode them,
sediments fill them in, and the tectonic recycling of Earth's
crust completely obliterates them," says Ocampo. On the
Moon, there are millions of well-preserved craters. On Earth,
"so far we've managed to find only about 174."

Sounds
like a job for Google. Seriously. Amateur astronomer Emilio
González pioneered the technique in March 2006. "I use
Google Earth," he explains. Google Earth is a digital
map of our planet made of stitched-together satellite images.
You can zoom in and out, fly around and inspect the landscape
in impressive detail. It's a bit like a video game—except
it is real.

González
began by calling up Kebira impact crater in Libya—the Sahara's
largest. It was so easy to see, he recalls, "I decided
to look around for more." Minutes later he was "flying"
over the Libya-Chad border when another crater appeared. And
then another. They both had multiple rings and a central peak,
the telltale splash of a high-energy impact. "It
couldn't be this easy!" he marveled.

Right:
The crater candidates González found are circled in red. [More]

But
it was. At least one of the craters had never been catalogued
before, and both, almost incredibly, lined up with the Aorounga
crater 200 km away: map.
In less than 30 minutes, González had found two good impact
candidates and possibly multiplied the length of the Aorounga
chain. Hours
of additional searching produced no new results. "Beginner's
luck," he laughs. (If you would like to hunt for your
own craters online, González offers these
tips.)

Ocampo
doubts that these new craters are related to Aorounga. "They
don't appear to be the same age." But she can't rule
it out either.

"We
need to do some fieldwork," she says. To prove a crater
is a crater—and not, say, a volcano—researchers must visit
the site to look for signs of extraterrestrial impact such
as "shatter cones" and other minerals forged by
intense heat and pressure. This kind of geological study can
also reveal the age of an impact site, marking it as part
of a chain or an independent event.

Answers
may have to wait. Civil war in Chad and the possibility of
war between Chad and Sudan prevent scientists from mounting
an expedition. Meanwhile, researchers are scrutinizing candidate
chains in Missouri and Spain. Although those sites are more
accessible than Chad, researchers still can't decide if they
are chains or not. It's difficult work.

Ocampo
believes it's worth the effort. "The history of Earth
is shaped by impacts," she says. "Crater chains
can tell us important things about our planet."